This invention relates to extrusion of profiled products in final form from powdered thermoplastic materials; and more particularly to directly extruding final products from powdered, or pulverulent, thermoplastic materials which have heretofore generally required an intermediate stage of plasticating compaction and granulation or pelletization before extrusion into final product configurations of good quality, such materials being particularly illustrated with acrylonitrile-butadiene-styrene (ABS) terpolymer compound.
Extrusion of products such as pipe directly from powdered plastic and even pelletized plastic often results in pores or voids in the product. This is particularly troublesome in a product such as extruded pipe, where the integrity of the wall is of paramount importance. This is believed to be caused by entrapped moisture or water, unpolymerized monomers, volatile oligomers, solvents and other rapidly volatilized constituents in the compound and blended plastic materials fed to the extruder. It has therefore generally been necessary in the operation of the conventional screw-type extruder, for the feed material to be present in a granule, chip, pellet, crumb or like form generally obtained by a first extrusion of a pre-plasticating type, or by other means such as roll milling, high intensity mixing and centrifuging and the like to partially cut down on the above-described defects, such as is set out in U.S. Pat. No. 2,746,086. As recited in that patent, pellets have many advantages over the pulverulent form in being more dense and therefore taking up less space; flowing more easily; less tendency to pick up foreign material or moisture and become contaminated; and in general by actual feed test will feed much faster by weight into a product extruder than pulverulent material would in otherwise equal conditions.
Thus for the manufacture of such thermoplastic materials, one must generally, to have a more marketable and processable product, pass it through a first extrusion into sheets or rods which are then chopped up or milled into pellets and the like forms, then sold to the formulator of finished products.
In most extruders, whether of the pre-plasticating type of intermediate pellet formulation for highly hygroscopic thermoplastics or of the final product configuration type, it has still been found that considerable devolatilizing must take place. Thus improvement in results has been achieved with extruders having vents or vacuum boxes along the length of the barrel to permit escape of or to draw out in gaseous form trapped moisture, unpolymerized monomer of a volatile nature, and the like readily volatilizable constituents. These constituents, including water and low boiling solvents, tend to cause porosity, bubbles, blisters, and similar defects in the extruded product and are particularly onerous in the finally extruded product. Thus it is customary to provide a decompression zone in the extruder screw; and at that location in the extruder barrel to provide a vent either to the atmosphere or to suction means for the partial elimination of such easily volatilized constituents. Various improvements on such devices have been suggested, for example that shown in U.S. Pat. No. 2,736,058 which provides a vertically adjustable baffle to be placed within the venting means so as to aid in preventing the flow of thermoplastic material through the vent along with the volatilized constituents. Another such improvement is that shown in U.S. Pat. No. 3,212,133 wherein means are provided for a vent duct unblocking mechanism for a screw extruder that is said to return escaping plasticized material to the extruder cylinder.
That patent further discloses the suggestion that it might be desirable to locate the vent duct under rather than above the barrel, when plasticized material that is particularly moisture retentive is employed, so that the condensation products which form after the vapors are cooled can be exhausted from the vent duct. Such a suggestion on first reading appears feasible but on reflection is considered, without elaborate preventive means, to be highly susceptible to having the plasticized extrudate flow into the venting area and plug the vent along with other disastrous results flowing therefrom.
Another approach in the intermediate extruding of thermoplastic materials for formulation into granular form is that indicated in U.S. Pat. No. 3,036,335 wherein vacuum venting of the barrel is extended back to and through the feed hopper/and supply drum, along with heating of these areas so as to partially degas the material entering the extruder. An insufficiency with this disclosed method is that it was not carried back far enough, particularly in the case of especially moisture-sensitive and sticky feed materials such as the ABS copolymers and the like. Also such systems have been plagued with vacuum and material leaks. Further it is virtually impossible to effectively pull sufficient gases back through the length of highly compressed viscous melt in the barrel.
Accordingly, it is one object and advantage of the present invention to provide a method and apparatus for the extrusion of powdered or pulverulent ABS directly into final product configuration, without the necessity of an intermediate stage as generally required for forming pellets of the ABS before configuration extrusion.
Another object and advantage is the provision of a method and apparatus for the extrusion of any powdered or pulverulent thermoplastic material, particularly of a sticky, hygroscopic nature and containing substantial quantities of moisture or water, unpolymerized monomers, volatile oligomers, solvents and other readily volatilized constituents, directly into extruded final product configurations without the intermediate stage generally required of forming pellets and the like thereof.
Still another object and advantage is the provision of a method and apparatus for improved devolatilizing of any thermoplastic suitable for melt extrusion by vacuum-equipped extruders into final product configurations.
Still another object is the provision of a method and apparatus for the extrusion of thermoplastic materials into extruded profiles, which method and apparatus eliminate heretofore generally required heat history steps which may have an adverse effect on hygroscopic heat degradational thermoplastic materials.
Still another object and advantage is the provision of means to induce condensation of volatiles removed from the extruder barrel and prevent them from finding their way back into the extrudate or into the atmosphere or the waste water system.
Other objects and advantages will become apparent through the ensuing disclosure.